The invention relates to thrust bearing assemblies and more particularly to such assemblies that are useful in supporting shafts of vertical electric motors on spherical roller thrust bearings in a manner that limits the maximum angle through which the housing washer of a given bearing can be tilted relative to a shaft supported on it.
Those familiar with loading requirements for thrust bearings have long known that spherical roller thrust bearings afford the most efficient support in certain applications on a dollar-per-pound-of-thrust basis. In general, such bearings are selected for use in applications where the combination of thrust loading and anticipated speed of rotation is between the relatively low speed, low thrust applications, for which angular contact ball bearings are suitable, and the higher speed, higher thrust applications for which Kingsbury bearings are more preferable. For example, one common application of spherical roller thrust bearings is in vertically mounted electric motors used to drive pumps. It is necessary in such an application to assure that the motor thrust bearing maintains the motor shaft in accurate alignment with a predetermined axis of rotation in order to prevent the rotor supported on the shaft from rubbing against the stator and to prevent other rotating parts from striking adjacent stationary parts. The applicant has found that undesirable misalignment of such a motor shaft can occur if the housing washer of a spherical roller thrust bearing supporting the thrust loading of the shaft becomes excessively tilted. Thus, controlling the tilt is important because excessive tilt causes more radial movement of the rotor shaft than is permitted by typical radial clearance between the bearing and its housing, particularly when the bearing is supported on springs as is common. As the housing washer part of the bearing tilts, it allows the rollers and shaft washer parts to move radially, while still contacting the housing washer raceway which is spherical in contour. It has been discovered that this radial movement, if not controlled by controlling the amount of tilt, can be great enough to cause excess vibration or cause striking between the rotating and stationary parts of the motor. It is important to realize that when such a spherical roller thrust bearing tilts, it causes a greater shift in the position of the shaft centerline than is permitted by the radial clearance between the housing washer and its seating support.
The risk of such undesirable tilting of the housing washer of a spherical roller bearing used as a thrust bearing for a vertical motor is substantially increased by the normal mode of operation of such motors and bearings. Due to the axial upthrust forces applied to such a motor by a pump when it is started, the shaft washer of the support bearing tends to rise away from its cooperating housing washer. Such bearing separation can persist for extended periods in some pumping applications when upthrust is applied to a supported motor shaft during a line filling operation. In order to prevent the bearing from becoming separated during this frequently encountered start-up phase of motor operation, it is common practice to spring-load the housing washer of the thrust bearing to continuously bias it toward the shaft washer, thereby to hold the parts of the bearing together when the supported shaft rises during start up or due to other forces applied to the shaft.
It is also common practice to limit the amount of upward movement of the shaft by adjusting the motor end play by arranging another bearing on the shaft that takes the upward thrust load. However, it is always necessary to retain some end play in such motors so that the bearings will not be loaded against each other during normal operation, accordingly, the risk of the thrust bearing being excessively tilted remains.
If there were no radial forces applied to a spherical thrust bearing when its housing washer is lifted off of its seat by biasing springs during light thrust load or start-up conditions of the motor, there would be little risk of the bearing tilting. However, the unbalanced magnetic pull on the motor shaft caused by the rotor being slightly off center in the motor, as well as by other unbalanced forces inevitably present in such motors, combine to exert some radial force on the thrust bearing. This unbalanced force tends to shift the shaft and tilt the bearing housing washer relative to its normal running seat. Such tilting action further increases the unbalanced magnetic force on the rotor and accentuates the risk of excessive bearing tilt. If a housing washer becomes sufficiently tilted during such a start-up or upthrust running condition during operation of a motor, it can either cause the rotor to rub on the stator or cause contact between other rotating and stationary parts and damage them. In other circumstances, where the degree of tilting of such bearings is slighter, the detectable undesirable symptom may be excessive vibration.
These major problems encountered in some applications of prior art spherical roller thrust bearings have long been recognized and in some vertical motor applications utilizing these bearings an attempt has been made to avoid rubbing and vibration problems by combining a separate guide bearing with it, at an axially spaced location on the supported motor drive shaft. Such a second bearing is designed to prevent the drive shaft from shifting unduly in a radial direction relative to its desired axis of rotation. That type of solution has been effective insofar as it serves to prevent the destruction of motors due to rubbing between adjacent mechanical parts or their rotor and stator assemblies; however, it is an expensive solution. Moreover, great care must be taken in the design, construction and assembly of such an arrangement to prevent the guide bearing from accepting thrust load which could lead to its early failure.
In view of the many applications where the use of spherical roller thrust bearings are desirable on a dollar-per-pound-of-thrust evaluation basis, it is apparent that an economical solution to the problem of thrust bearing tilting is very beneficial.